get data from another cell only once

[andrewb90]

I have two cells that give me a percentage. One is a cell that is the average add on sales percentage for the month and changes every day as new info is entered. On each day of the month I have one cell that reflects the current percentage calculated on another sheet and the other is the difference between the current average and the days average.
I want to have the difference calculated when the daily paperwork is filled out but do not change every day when the overall average will change. Basically only looking at that cell once and then not changing after.

Can anybody help me with this?

Thanks,

Andrew